Newly promoted Tucuman, from the country's north, is in fourth place. The team has played only two seasons in the top flight over the past two decades, both times being relegated straightaway.
Boca Juniors has made a better start than its rival, but lost its first game of the season, 3-2 at home to CA Tigre. Tigre is another one of the league's surprise early front-runners, sitting in second place behind San Lorenzo.
Tigre's official website called the win at Boca "an unforgettable triumph." The Buenos Aires-based team, which finished 14th in last season's closing championship, took a 2-0 lead after just 12 minutes. Boca came back to level quickly, but the game's scoring was over by just past the half-hour mark when Tigre's Martin Morel scored his second of the game.
Boca is placed fifth and has a game in hand over the four teams above it. However, the team's form took a dip after winning its first three games. The previous weekend it was held at home by a nine-man Independiente side, and then in midweek could only tie 0-0 at lowly Argentinos Juniors.